There have been typically used releasing films having a cured silicone resin layer formed thereon. They are used in a variety of application fields such as a carrier film used in the production of molded article or as a protective film for an adhesive layer or for a plaster, or the like. In respective fields where the conventional releasing films are now used, there have been demanded releasing films that have further improvement in properties such as releasability, dimensional stability, transfer of impurities, gas permeability or the like.
The use of the releasing films in some application fields and their properties required will be described below.
In the production of a molded article, the releasing film is used as a carrier film in molding of a resin sheet, a resin coat, a ceramic sheet or the like.
The resin sheet is molded by coating (casting) a resin solution containing, for example, vinyl chloride or the like, on a carrier film, heating the resulting carrier film to remove a solvent therefrom, and peeling off and separating the carrier film. It is used in applications such as a marking sheet.
The releasing film is used also for laminating and molding a prepreg in the production of a printed wiring base board. That is, a solution of a thermosetting resin such as a phenolic resin or an epoxy resin is coated directly on a releasing film or is impregnated into a glass fiber cloth or paper sheet laminated on the releasing film and subsequently, the resulting releasing film is heated to remove the solvent therefrom. The releasing film is then peeled off and separated to produce a prepreg.
The resin coat is produced, for example, by coating a carrier film surface with a coating solution obtained by dissolving a resin as an adhesive in a solvent and then heating the resulting carrier film to remove the solvent.
The ceramic sheet is produced as a molded article, for example, by coating a carrier film surface with a slurry obtained by dispersing a ceramic powder, a binder or the like in a solvent, heating the resulting carrier film to remove the solvent, and then peeling and removing the carrier film.
In general, out of surfaces of a resin sheet, ceramic sheet or the like, the surface from which a carrier film has been peeled off is used frequently for applications which must have surface properties of high precision. Therefore, the surface roughness of the base film used in the carrier film has a large influence on the surface roughness of the resin sheet, ceramic sheet or the like.
As a base film of the above carrier films, there are used a variety of films, particularly a biaxially oriented polyethylene terephthalate (may be abbreviated as PET hereinafter) film. The PET film usually has its surface roughened by incorporation of a filler or the like, for the improvement of its windability. Consequently, a resin sheet, a ceramic sheet and the like, which are produced by the use of a carrier film with a filler-containing PET film as a base film, has a rough surface and, when laminated in a plurality of layers, may contain gaps between the adjacent layers.
On the other hand, the releasing film is heated to remove the solvent after it is coated with a resin solution or ceramic slurry. The heating temperature is, in many cases, close to or higher than the glass transition temperature (Tg) of the base film used in the releasing film. As a result, a problem arises that thermal deformation such as dimensional change and wrinkles occurs on the releasing film and the molded resin sheet or the like has non-uniform thickness and poor surface smoothness, thereby lowering the quality of the product. It is feared that the above problem may be more manifest as the heating time is shortened and the heating temperature is raised to increase the productivity of the resin sheet or the like.
The releasing film is used also in an another application field where high releasability is required and the transfer of the impurities to the releasing layer must be considered. An example of such an application field includes; the use of the releasing film as a protective film for an adhesive film used in the back-grinding step or dicing step of silicon wafer production.
That is, the adhesive film in which a variety of adhesive layers are laminated is used to fix a wafer in the back-grinding step or dicing step of silicon wafer production.
Particularly, in the back-grinding step, the side of the wafer on which circuits are printed is fixed by the adhesive layer, then its back surface is polished, and the adhesive layer is peeled off. Therefore, even a small amount of a residual adhesive after peeling or the transfer of impurities from the protective film for the adhesive layer to the wafer surface causes problems such as a defective printed circuit or the like and lowers the yield of semiconductor chips.
In the dicing step, a silicon wafer, while being fixed by an adhesive layer, is cut into small pieces, UV irradiation or the like is then made to reduce the adhesivity of the adhesive layer, and each piece is taken out. Each of the resulting wafer pieces is transferred to the subsequent bonding step and molding step for use therein. The adhesive film is stored usually in a state that its adhesive layer surface is protected by a releasing film, and is used in the dicing step after peeling and removing the releasing film.
In recent years, the size of silicon wafer used in the molding step has become very small. Presence of even a small amount of impurities on the wafer causes, for example, wafer cracking starting from the impurities due to the poor adhesion, resulting in lowering the yield of product.
Meanwhile, as the above protective film for adhesive layer, there has been used, for example, a releasing film comprising a polyester film and a cured silicone releasing layer formed thereon. Such a releasing film, however, has a drawback in that the impurities (oligomers and a metal compound used as a polymerization catalyst) in the polyester film tend to transfer into the silicone releasing layer and deposit on the releasing layer. The impurities deposited on the releasing layer are transferred onto the adhesive layer surface of the adhesive film and are further transferred onto a silicon wafer in the back-grinding and dicing steps of silicon wafer production, whereby reduction in the yield of product in the molding step is caused. Further, presence of impurities (uncured silicone monomer or oligomers) in the cured silicone resin constituting the releasing layer of the releasing film reduces the yield as well.
A releasing film is used also in other application field, i.e. a medical treatment field. For example, a releasing film is used as a protective film for protecting the adhesive layer of a plaster and maintaining the efficacy of the medicinal component contained in the adhesive layer. In such a medical treatment field, as a plaster which scarcely causes rash and which supplies a clinically effective amount of a medicine to the diseased part, there have been known, for example, plasters using, as a constituent, a knit of finely porous hollow fiber (WO 087/00046, WO 087/04343 and WO 090/09784). A plaster is also known which is a laminate of said knit and a very thin polyester film (Japanese Patent Application Kokai (Laid-Open) 3-816044).
As the protective layer for the medicinal component-containing adhesive layer of a plaster, a releasing paper using paper as the base material has been used. This releasing paper, however, has had a drawback in that since paper is porous, the medicinal component of the plaster bleeds out or volatilizes gradually, resulting in shortening of effective period of the medicinal component. To alleviate the drawback, a polyethylene terephthalate (PET) film having a higher gas barrier property than paper is also in use as the base material for the above protective layer. However, a releasing film having higher gas barrier properties is currently desired in order to make longer the effective period of medicinal component.